The invention relates to a method of producing a fracture-divided component, particularly a connecting rod, having an at least partially coated bore. The invention also relates to a component produced according to the method.
In the case of components which have no separate sliding or rolling bearings, such as integral connecting rods for motor vehicle internal-combustion engines, it is known to temper or to coat the bearing surfaces of the components, so that the latter withstand high stress. Likewise, it is known to produce bearing components in one pass and only subsequently separate one bearing half by fracture-division if the later assembly requires a divided component (for example, connecting rod big end and connecting rod bearing cap).
German Patent documents DE 10 2004 003 403 A1 and DE 10 2004 018 921 A1 describe manufacturing methods by which at least the large bearing eye of a connected rod is coated before it is fracture-divided. In the case of these methods, it is assumed that, after the thermal coating with a high-melting bearing material (for example, copper-(Cu—) based, permissible component or substrate temperature>200° C. after the finishing of the coating), no or only negligible internal tensions are present in the actual connecting rod as well as in the substance-bonded coating, so that the fracture-divided component halves can be assembled again in a precisely fitting manner without any straightening operation or auxiliary assembly tools.
However, tests have shown that, after the coating of bearing components with low-melting bearing materials (such as tin antimony alloys (SnSb) systems, permissible component or substrate temperature<100° C. after the finishing of the coating), internal tensile stresses are present in the coating, which are released when fracture-dividing the bearing halves. The therefore occurring varying degrees of distortion of the bearing halves because of different rigidities have the result that, for a precisely fitting reassembly, the bearing halves either have to be straightened during a finishing or during the mounting, or that auxiliary tools are required.
Analogously, the same applies to high-melting bearing materials and an occurring component or substrate temperature of <200° C. after the finishing of the coating.
From German Patent document DE 10 2007 021 622 A1, a method is known for producing a fracture-divided bearing component of carbon steel, where a tempering and/or coating of a bearing surface takes place at least in sections, which tempering and/or coating is connected with the introduction of tensions. Before or after the fracture-dividing, a partial heat treatment for a targeted introduction of counter-tensions or a working is therefore provided. In this case, it is assumed that, when fracture-dividing a connecting rod made of carbon steel that is provided with a copper-based coating, there will be varying degrees of “spring-back” of the bearing halves (connecting rod big end and connecting rod bearing cap). Spring-back means that the overall dimension (width measurement) of a bearing half will be larger after the fracture division than before the fracture division. A spring-back will occur when there is, for example, a martensite structural transformation (increase in volume as a result of martensite formation) in the bore area close to the surface.
It is an object of the invention to provide an advantageous method of producing a fracture-divided component, whereby, particularly in the case of a coating with a low-melting material, it is ensured that the two component halves can be reassembled in a precisely fitting manner after the fracture division.
According to a first aspect of the invention, a method for producing a fracture-divided component, particularly a connecting rod, having an at least partially coated bore, comprises the following steps:
providing an unfinished component;
applying a coating of a bearing material to a bore surface; and
working the coating in order to compensate internal tensile stresses in the coating before a fracture division of the component.
The invention is based on the recognition that, particularly in the case of a coating of a component with a low-melting bearing material, for example, with an Sn-based powder, tensile stresses are induced in the coating which result in a “falling-in” after the fracture division. The overall dimension (width measurement) of a bearing half will therefore be smaller after the fracture division than before the fracture division. In the concrete case of a connecting rod that is fracture-divided at the large connecting rod eye, the coated connecting rod bearing cap will therefore fall in compared with the uncoated connecting rod big end because of different rigidities. The invention counteracts this undesirable effect in that the coating is mechanically worked before the fracture division of the component in order to compensate the internal tensile stresses in the coating.
The working preferably includes a rolling, particularly a deep rolling of the coating.
The internal tensile stresses in the coating can additionally be counteracted in that, before the working, the coating is reduced to a defined measurement within the scope of a material-removing intermediate machining.
In the case of bearing components exposed to maximum stress, the working (rolling) can intentionally be increased to such an extent that inherent compressive stresses remain in the coating. In addition to the working of the coating, a counter-hardness zone can be formed for generating counter-compressive stresses which oppose the internal tensile stresses of the coating.
The counter-hardness zone is preferably formed before the application of the coating and preferably on the outer contour of the component. The size, shape and position of the counter-hardness zone can be determined experimentally.
According to a second aspect of the invention, a method for producing a fraction-divided component, particularly a connecting rod, having an at least partially coated bore, comprises the following steps:
providing an unfinished component;
applying a coating of a bearing material to a bore surface before a fracture division of the component; and
forming a counter-hardness zone before the application of the coating in order to generate compressive stresses which oppose the internal tensile stresses of the coating.
In the case of this alternative method, the tensile stresses induced in the coating, which result in a “falling-in” after the fraction division of the component, are counteracted just by forming a counter-hardness zone before the coating is applied. The counter-hardness zone generates compressive stresses in the component, which oppose the internal tensile stresses of the coating.
The counter-hardness zone is preferably formed on the outer contour of the component. The size, shape and position of the counter-hardness zone can, in turn, be determined experimentally.
The invention thus also provides a component, particularly a connecting rod, produced according to the method of the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.